The present invention relates to a heat-shrinkable laminate film comprising a core layer made of a saponified copolymer of ethylene and vinyl acetate, two outer layers made of a material selected from mixtures of a copolymer of ethylene and vinyl acetate and a linear low-density polyethylene or a copolymer of propylene and ethylene and two adhesive layers, the ratio of the weight of the copolymer of ethylene and vinyl acetate to the total weight of the polymers of the two outer layers being higher than 0.4, more particularly relates to a heat-shrinkable laminate film excellent in gas-barrier property, oil-resistance and peel-resistance of the sealed parts in the heat shrinkage (hereinafter referred to as peel-resistance), comprising a core layer made of a saponified copolymer of ethylene and vinyl acetate, two outer layers made of a resin selected from mixtures of 80 to 30% by weight of a copolymer of ethylene and vinyl acetate having a crystal melting point in a range of 85.degree. to 103.degree. C. and 20 to 70% by weight of a linear low-density polyethylene having a crystal melting point in a range of 110.degree. to 125.degree. C. or a copolymer of propylene and ethylene having a crystal melting point of 126.degree. to 155.degree. C., and two adhesive layers disposed between the core layer and the outer layers, the ratio of the weight of the copolymer of ethylene and vinyl acetate to the total weight of the polymers of the two outer layers being larger than 0.4.
For packaging irregularly shaped fatty foodstuffs such as raw meats, processed meats, cheeses, etc., a heat-shrinkable packing film material is most simple and convenient. Since such a packaged foodstuff is required to be preserved safely for a long time period, not only the gas-barrier property but also the oil-resistance and the peel-resistance are required for the film material used in such heat-sealing packaging.
Namely, in the case where fatty foodstuffs are packaged in such a film material and the thus packaged foodstuffs are sterilized, there has been frequently observed that the film softened by the fats and the heat is thinly stretched resulting in breaking or that the thus softened film is broken at the sealed part or the neighborhood thereof by the heat-shrinking stress generated during the sterilization. Accordingly, a heat-shrinkable film having the excellent gas-barrier property, the high oil-resistance and peel-resistance have been demanded from the field of food-packaging industry.
Hitherto, as a heat-shrinkable film having the excellent gas-barrier property, in which a saponified copolymer of ethylene and vinyl acetate is used as the gas-barrier layer, the laminate film comprising polyolefin, a saponified copolymer of ethylene and vinyl acetate and a polyamide (hereinafter referred to as the laminate film of polyolefin/polyamide/saponified copolymer of ethylene and vinyl acetate) disclosed in Japanese patent application laying open No. 56-136365 (1981) and the laminate film comprising an ionomer, a saponified copolymer of ethylene and vinyl acetate and a polyamide or a copolymer of ethylene and vinyl acetate (hereinafter referred to as the laminate film of ionomer/saponified copolymer of ethylene and vinyl acetate/polyamide or the laminate film of ionomer/saponified copolymer of ethylene and vinyl acetate/copolymer of ethylene and vinyl acetate) disclosed in Japanese patent application laying open No. 56-89444 (1981), etc. have been known.
In the preparation of the laminate film of polyolefin/polyamide/saponified copolymer of ethylene and vinyl acetate, the adhesion of the polyolefin layer and the polyamide layer is carried out by an extra-die adhesion method, and accordingly it is difficult to quench the material, and so, the crystallization of polyolefin proceeds to give a bad effect on the stretchability of the product. In addition, in the case of using such a resin (for instance, an ordinary polyethylene or a copolymer of propylene and ethylene) of a high crystal melting point and of a high crystallizability, the temperature for stretching becomes high and it is impossible to process such a resin into a heat-shrinkable film of a heat-shrinkage of larger than 15% at 90.degree. C.
On the other hand, in the case where a copolymer of ethylene and vinyl acetate of a relatively low in crystallization is used, the oil-resistance and the peel-resistance are reduced and the thus prepared laminate film is unsuitable for packaging materials containing oils and fats such as fatty foodstuffs. In addition, because of the hygroscopic property of the saponified copolymer of ethylene and vinyl acetate and the polyamide, the gas-barrier property of the saponified copolymer of ethylene and vinyl acetate is remarkably reduced during heat-shrinkage and sterilization which are carried out while utilizing a hot water, the fact being a problem.
In the laminate film of ionomer/saponified copolymer of ethylene and vinyl acetate/polyamide, since the ionomer is also hygroscopic as well as the polyamide, there are cases where the layer of saponified copolymer of ethylene and vinyl acetate becomes a hygroscopic state indirectly via the layer of polyamide resulting in the reduction of the gas-barrier property of the laminate film.
In addition, in the case where the ionomer is used for preparing the inner layer (one of the outer layers of the laminate film, and becomes the inner layer after the packaging has been completed), it occasionally reduces the peel-resistance.
The laminate film of ionomer/saponified copolymer of ethylene and vinyl acetate/copolymer of ethylene and vinyl acetate causes frequently the problem of poor oil-resistance, poor peel-resistance and poor gas-barrier property as the laminate films known.
As stated above, in the case where a low-density polyethylene or an ionomer is used for preparing both of the outer layers, particularly the peel-resistance has been insufficient. On the other hand, in the case where a copolymer of ethylene and vinyl acetate is used for the purpose, the oil-resistance of the laminate film has been insufficient. In another case where a polymer of a high crystal melting point such as copolymer of ethylene and propylene and polypropylene is used as the sole material for the outer layer, although the peel-resistance and the oil-resistance of the laminate film is improved, it is difficult to stretch the layer for obtaining the heat-shrinkage at 90.degree. C. of higher than 15% because of the lack of stretchability of such a polymer.
In addition, even in the case where the layer of a saponified copolymer of ethylene and vinyl acetate is laminated with a layer of a hygroscopic resin, the gas-barrier property of the thus prepared laminate film is frequently reduced during the heat-shrinkage and the sterilization.
Accordingly, the development of a heat-shrinkable laminate film excellent in oil-resistance, peel-resistance and gas-barrier property has been keenly demanded in the field of food-packaging industry.